The present invention relates to gas turbine engine combustors and, more particularly, to can-annular combustors with pre-mixers.
Industrial gas turbine engines include a compressor for compressing air that is mixed with fuel and ignited in a combustor for generating combustion gases. The combustion gases flow to a turbine that extracts energy for driving a shaft to power the compressor and produces output power for powering an electrical generator, for example. Electrical power generating gas turbine engines are typically operated for extended periods of time and exhaust emissions from the combustion gases are a concern and are subject to mandated limits. Thus, the combustor is designed for low exhaust emissions operation and, in particular, for low NOx operation. A typical low NOx combustor includes a plurality of combustor cans circumferentially adjoining each other around the circumference of the engine. Each combustor can has a plurality of pre-mixers joined to the upstream end. Lean burning pre-mixed low NOx combustors have been designed to produce low exhaust emissions but are susceptible to combustion instabilities in the combustion chamber.
Diatomic nitrogen rapidly disassociates at temperatures exceeding about 3000.degree. F. and combines with oxygen to produce unacceptably high levels of NOx emissions. One method commonly used to reduce peak temperatures and, thereby, reduce NOx emissions, is to inject water or steam into the combustor. However, water/steam injection is a relatively expensive technique and can cause the undesirable side effect of quenching carbon monoxide (CO) burnout reactions. Additionally, water/steam injection methods are limited in their ability to reach the extremely low levels of pollutants required in many localities. Lean pre-mixed combustion is a much more attractive method of lowering peak flame temperatures and, correspondingly, NOx emission levels. In lean pre-mixed combustion, fuel and air are pre-mixed in a pre-mixing section and the fuel-air mixture is injected into a combustion chamber where it is burned. Due to the lean stoichiometry resulting from the pre-mixing, lower flame temperatures and NOx emission levels are achieved. Several types of low NOx emission combustors are currently employing lean pre-mixed combustion for gas turbines, including can-annular and annular type combustors.
Can-annular combustors typically consist of a cylindrical can-type liner inserted into a transition piece with multiple fuel-air pre-mixers positioned at the head end of the liner. Annular combustors are also used in many gas turbine applications and include multiple pre-mixers positioned in rings directly upstream of the turbine nozzles in an annular fashion. An annular burner has an annular cross-section combustion chamber bounded radially by inner and outer liners while a can burner has a circular cross-section combustion chamber bounded radially by a single liner.
Industrial gas turbine engines typically include a combustor designed for low exhaust emissions operation and, in particular, for low NOx operation. Low NOx combustors are typically in the form of a plurality of combustor cans circumferentially adjoining each other around the circumference of the engine, with each combustor can having a plurality of pre-mixers joined to the upstream ends thereof. Each pre-mixer typically includes a cylindrical duct in which is coaxially disposed a tubular centerbody extending from the duct inlet to the duct outlet where it joins a larger dome defining the upstream end of the combustor can and combustion chamber therein.
A swirler having a plurality of circumferentially spaced apart vanes is disposed at the duct inlet for swirling compressed air received from the engine compressor. Disposed downstream of the swirler are suitable fuel injectors typically in the form of a row of circumferentially spaced-apart fuel spokes, each having a plurality of radially spaced apart fuel injection orifices which conventionally receive fuel, such as gaseous methane, through the centerbody for discharge into the pre-mixer duct upstream of the combustor dome.
The fuel injectors are disposed axially upstream from the combustion chamber so that the fuel and air has sufficient time to mix and pre-vaporize. In this way, the pre-mixed and pre-vaporized fuel and air mixture support cleaner combustion thereof in the combustion chamber for reducing exhaust emissions. The combustion chamber is typically imperforate to maximize the amount of air reaching the pre-mixer and, therefore, producing lower quantities of NOx emissions and thus is able to meet mandated exhaust emission limits.
Lean pre-mixed low NOx combustors are more susceptible to combustion instability in the combustion chamber which causes the fuel and air mixture to vary, thus, lowering the effectiveness of the combustor to reduce emissions. Lean burning low NOx emission combustors with pre-mixers are subject to combustion instability that imposes serious limitations upon the operability of pre-mixed combustion systems. There exists a need in the art to provide combustion stability for a combustor which uses pre-mixing.